1. Field of the Invention
The present invention relates to a permanent magnet motor, and more particularly to a structure of the permanent magnet motor with higher performance in which the cogging torque is reduced and the torque ripple is also reduced.
2. Description of the Related Art
In recent years, there is a demand for electric motors having small cogging torque and torque ripple for various purposes. Conventionally, one method of reducing the cogging torque of a permanent magnet motor has been proposed in which auxiliary grooves are formed on a face of each magnetic pole piece of the stator opposed to the permanent magnets of the rotor, though not shown, as disclosed in JP-A-10-42531, or a skew is provided by the amount of a period of cogging torque, as disclosed in JP-A-8-223832, for example.
In the conventional permanent magnet motor, as described above, the auxiliary grooves are in a face of each magnetic pole piece of the stator, opposed to the permanent magnets of the rotor, and a skew is provided in the amount of a period of the cogging torque, to reduce the cogging torque. However, when a skew angle is inappropriate to reduce the torque ripple, the cogging torque and the torque ripple cannot be reduced concurrently. For example, in a six-pole, nine-slot electric motor, with two auxiliary grooves in a face of each magnetic pole piece of the stator, there is almost the equivalent effect as if the number of slots in the stator is 27. Then, the cogging torque ripples per rotation of the rotor is equal to 54, which is the smallest integer having the number of poles, 6, and the virtual number of slots, 27, as factors. The period of cogging torque is converted to 20 degrees in terms of the electrical angle, and the cogging torque can be significantly reduced by an electrical angle skew of 20 degrees.
However, in order to reduce the torque ripple, the fifth and seventh harmonics of the induced voltage must be reduced.
Since the induced voltage usually contains more of the fifth harmonic than of the seventh harmonic, it is effective, for reducing the torque ripple, to provide a skew at an angle where the fifth harmonic can be fully eliminated. However, the fifth harmonic cannot be reduced if the skew is provided at an electrical angle of 20 degrees, as described above. Therefore, in the case where this electric motor is driven by a sinusoidal electric current, the torque ripple does not become small. This is a problem because even if the auxiliary grooves or the skew is provided, it has been difficult to reduce the cogging torque and the torque ripple.